[Why]

I was wondering why REF is set to time delayed tripping of around 200 ms as REF will only cause tripping when the part of winding has grounded and obviously REF wont trip for imbalance and ….
I was wondering why REF is set to time delayed tripping of around 200 ms as REF will only cause tripping when the part of winding has grounded and obviously REF wont trip for imbalance and other factors which causes other type of differential relays to trip?In that case it should be set to instantaneous.isnt it?
Secondly, i want to know that if high impedance differential is employed for over-coming C.T saturation problem,Miss-match b/w CT's and different magnetizing curves of C.Ts, Same problems can be faced in simple or low impedance differential as well.Then why main and other differential relays are not set as high impedance too?

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[Hamid]

I don

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